1. Field of the Invention
The present invention relates to an AC-DC converter that receives an alternating-current voltage and outputs a direct-current voltage. In particular, the present invention relates to a PFC converter that improves the power factor.
2. Description of the Related Art
Japan, Europe, and so on have exercised harmonic current regulations where apparatuses are classified depending on the application, input power, and the like. To address these regulations, there has been carried out an idea that a circuit called a PFC (power factor correction) converter is added to the power supply of a general home electrical appliance to which the regulations apply, so as to suppress harmonic currents.
A general switching power supply apparatus using a commercial alternating-current power supply as an input power supply rectifies and smoothes a commercial alternating-current power supply to convert it into a direct-current voltage and then switches the direct-current voltage in a DC-DC converter. Thus, the input current becomes discontinuous and distorted from a sine wave, resulting in harmonic currents.
To suppress such harmonic currents, a PFC converter is provided after a full-wave rectifier circuit and before a smoothing circuit composed of a smoothing capacitor.
This PFC converter is composed of a chopping circuit and operates so that the waveform of the input current has a shape similar to the waveform of the input voltage, that is, the waveform of the input has an in-phase sinusoidal shape. Thus, harmonic currents are suppressed to a certain level or below.
Hereafter, an example configuration of the PFC converter shown in Japanese Unexamined Patent Application Publication No. 2004-282958 will be described on the basis of FIG. 1.
In a power factor correction circuit shown in FIG. 1, a series circuit composed of a switching element Q1 and a current detection resistor R is connected to both output terminals of a diode bridge B1, which rectifies the alternating-current voltage of an alternating-current power supply Vac. The switching element Q1 is composed of a boosting reactor L1 and a MOSFET. A series circuit composed of a diode D1 and a smoothing capacitor C1 is connected to both end portions of the switching element Q1, and a load RL is connected to both end portions of the smoothing capacitor C1. The switching element Q1 is configured to be turned on or off under the PWM control of a control circuit 10. The current detection resistor R detects an input current flowing into the diode bridge B1.
The control circuit 10 includes an error amplifier 11, a multiplier 111, an error amplifier 113, a voltage control oscillator (VCO) 115, and a PWM comparator 116.
The error amplifier 111 obtains the error between the voltage of the smoothing capacitor C1 and a reference voltage E1. The multiplier 112 multiplies the error voltage signal by the voltage rectified by the diode bridge B1. The error amplifier 113 generates an error between the result of the multiplication performed by the multiplier 112 and the current signal flowing into the diode bridge B1, and outputs the error to the PWM comparator 116.
The VOC 115 generates a chopping signal with a frequency corresponding to the rectified voltage value of the alternating-current power supply voltage.
The PWM comparator 116 receives the chopping signal from the VCO 115 via the − terminal thereof, as well as receives the signal from the error amplifier 113 via the + terminal thereof. That is, the PWM comparator 116 provides, to the switching element Q1, a duty pulse corresponding to the current flowing into the diode bridge B1 and the output voltage of the diode bridge B1. This duty pulse is a pulse width control signal that compensates for variations in the alternating-current power supply voltage and the direct-current load voltage continuously in a certain period. Such a configuration performs control so that the current waveform of the alternating-current power supply matches the voltage waveform of the alternating-current power supply, improving the power factor.
Japanese Unexamined Patent Application Publication No. 7-177746 discloses a PFC converter that performs digital control.
Also in the case of digital control, current flowing into the inductor is detected, and the switching element is switched under PWM control corresponding to the value of the detected current.
As described above, a PFC converter is basically required to detect current passing through an inductor (hereafter referred to as “the inductor current”) in order to make the shape of the input current similar to the waveform of the input voltage.
Typical current detection methods include the following methods:
(1) Insertion of a current detection resistor into the current path in series to detect a fall voltage generated at both end portions of the resistor;
(2) Detection of current by insertion of a current transformer into the current path or by use of a current transformer having an inductor on its primary side; and
(3) Disposition of a Hall sensor on the current path to detect the output voltage.
In the above-mentioned method (1) using a current detection resistor, power consumption in the current detection resistor constitutes an equivalent loss. This is a problem in terms of loss reduction. In the method (2) using a current transformer, the direct-current components of current to be detected are cut off. Thus, only the alternating-current components of the current can be detected and the direct-current components (DC offset) thereof cannot be detected. Detecting each current using a current transformer to synthesize a signal allows detection of the inductor current; however, two current transformers are required in this case. The method (3) using a Hall sensor does not cause the problems with the above-mentioned methods (1) and (2); however, it is disadvantageous in that the cost as a whole is increased since the sensor is expensive.
Particularly, in a case where an AD converter is used in a digital-control PFC in order to detect current, as in the PFC converter according to Japanese Unexamined Patent Application Publication No. 7-177746, it is difficult to continue obtaining the value continuously, owing to the clock, the sample hold time, the number of resistors, and the like. This makes it difficult to obtain the average value from the inductor current containing a ripple. Moreover, elimination of a ripple using a filter involves insertion of the filter into the control loop system, resulting in the degradation of responsiveness.